This invention relates to a single subassembly which is adapted to perform all of the registration, image transfer and pickoff functions in an electrophotographic copier.
Electrophotographic copiers of the image transfer type, commonly referred to as plain paper copiers, are well known in the art. In copiers of this type, a repetitively used photoconductive surface such as that of a conductive drum coated with selenium is uniformly electrostatically charged and then is exposed to a light image of an original to form an electrostatic latent image. The surface then moves through a developing station at which a suitable developer is applied to the surface to form a developed toner image and to a transfer station at which the developed image is transferred to a copy sheet, which is usually a sheet of plain paper.
In at least one type of plain paper copier known in the art, the copy sheet to which the image is to be transferred is advanced to the nip between a pair of registration rollers spaced from the photoconductor drum and stops. In response to movement of the machine scanner, the registration rollers are driven to advance the copy sheet to the transfer station so that the leading edge thereof arrives at the transfer station in synchronism with the arrival of the leading edge of the image on the photoconductor. After the image has been transferred to the copy sheet, the sheet moves with the photoconductive surface to a pickoff station at which the leading edge of the sheet is stripped from the surface of the photoconductor and the sheet is carried away for delivery to the customer. U.S. Pat. No. 4,009,957 shows a copying machine of the type described.
It will be appreciated that each of the individual image transfer and pickoff systems of the type described above requires a certain amount of space around the photoconductive drum. Where an attempt is made to reduce the drum diameter thus to reduce the cost of a copying machine, the limiting factor most likely will be the amount of equipment which must be positioned adjacent to and around the drum to perform the required operations. Clearly, the provision of separate assemblies for transfer and for pickoff limits the extent to which the size of the drum can be reduced. Obviously, also, the registration rollers occupy space in the machine cabinet.
In addition to the foregoing, pickoff assemblies of the prior art suffer from a number of defects. Mechanical devices such as blades and the like which are adapted to lift the leading edge of the copy sheet off the drum, while simple, are relatively unreliable since they must overcome not only the mechanical adhesion between the paper and wet imaging surface but also electrostatic forces which usually are present. As an alternative mechanical expedient, it has been suggested that a stationary strip be placed at the edge of the drum at the transfer station so that a side of a copy sheet fed to the drum at the transfer station is received by the strip to maintain a separated paper edge. This expedient, however, reduces the effective width of the copy sheet and may result in loss of a part of the information on the original.
My co-pending application, Ser. No. 085,689 filed Oct. 17, 1979, for a Combined Transfer and Registration System for Electrophotographic Copier discloses an arrangement in which transfer of the developed image from the surface of the photoconductor to the transfer medium is effected by a roller which is spaced from the photoconductor by a distance which is less than the thickness of a sheet of copy material and which has a hardness approximately that of the copy material. In addition the transfer roller of the apparatus shown in my co-pending application cooperates with another roller to form a nip thus to perform the additional function of acting as a registration roller. Not only does this system achieve a most effective image transfer even where relatively rough copy material is used but also it appreciably simplifies the apparatus over that described above.